Duplex stainless steel is becoming more widely used in applications that require high strength and corrosion resistance. A typical example is deep sea applications in the oil and gas industry. Particularly for high pressure operations, thick walled or heavy duplex tubing may be required. By “thick walled” or “heavy” tubing, which are used interchangeably herein, is meant duplex tubing greater than 2 mm wall thickness. For most applications, duplex tubing is welded either to additional sections of duplex tubing or to fittings, valves and so forth all of which may be made of duplex steel.
Duplex steel is characterized by a phase balance between austenite and ferrite in the steel crystalline structure. In general, duplex stainless steels contain about 30 to 70 vol. % ferrite, more typically about 35 to 60 vol. % ferrite, even more typically about 40 to 45 vol. % ferrite, with the balance being austenite. Maintaining the austenite/ferrite phase balance is very important in that the austenitic phase contributes to the pitting corrosion resistance of the steel while the ferrite phase contributes to greater strength and resistance to chloride stress corrosion cracking.
Welding of duplex steels presents special challenges, since a proper phase balance and nitrogen content must be maintained in the weld metal as well as in the surrounding heat affect zone (HAZ). This is because welding subjects the material forming the weld bead to additional high heat, melting, cooling and solidification. Many factors associated with the welding process can affect the phase balance in the weld metal. These factors include the welding temperature, cooling rate, type of purge gas used during the welding operation and the chemistry of the weld pool. If the final weld solidifies with too much austenite, the strength of the weld can be compromised. If the weld solidifies with too much ferrite, the weld and HAZ may exhibit lower corrosion resistance.
Welding of steel tubing is done both manually and by machine. In both operations, sagging or drop through of the weld pool should be avoided so that the weld bead is uniform in profile along its entire length. In addition, the weld bead should not be too wide in profile, since a weld bead which solidifies too slowly may exhibit improper metallurgy in terms of both chemistry and phase structure. Incomplete penetration through the tube wall should also be avoided.
Machine welding of steel tubing is typically done commercially using orbital welding equipment in which heat for welding is derived from an electric arc generated by a pulsed electric current. The arc emanates from an electrode positioned outside the tubing adjacent the weld junction to be formed, with the electrode being moved by the machine orbitally (circumferentially) around the tubing along its entire circumference. Preferably, orbital welding is accomplished in a single pass (plus an additional 30° to 120° in some instances to complete the weld smoothly), since this approach minimizes problems occurring when a previously formed weld bead is remelted.
Thick walled duplex tubing is particularly difficult to weld, since the factors causing poor weld profile and improper phase balance magnify as tube wall thickness increases. Therefore, it has not been possible to achieve acceptable weld quality when machine welding duplex steel tubing of heavy wall thickness. Although manual welding can achieve acceptable weld quality, a highly skilled welder is required. Moreover, multiple weld passes are also required, which only exacerbates the complexity and expense of the welding process.
It is, therefore, an object of the present invention to provide welding apparatus and methods that significantly improve the weldability of duplex stainless steel by producing acceptable weld profiles and weld beads exhibiting a proper duplex phase balance and nitrogen retention.
It is a further object of the present invention to provide a welding process and apparatus that facilitate machine based welding, especially single pass orbital welding, of duplex stainless steel tubing, especially thick walled duplex stainless steel tubing.